Various screening tasks require massive computing capabilities. Although computing devices have shown ever increasing processing power, there is still a need for high speed computing, especially when it comes to the screening of images. Optical correlators could eventually fill the gap between the applications and the processing requirements.
An optical correlator takes advantage of the powerful capabilities of light to perform real-time computation. As illustrated in FIG. 7 (Prior Art), a light beam incoming from a laser source is directed through a first set of lenses to expand its diameter. The light passes through a first spatial light modulator on which an image is displayed. Then, the modulated beam will undergo a first Fourier transform by passing through another lens. The Fourier transform is performed simply by the propagation of the light and as such is realised very rapidly.
It is an inherent property of a lens to perform a Fourier transform on an input image that will be observed at the front focal plane of the lens, provided that this image is displayed at the back focal plane of the lens. The optically-computed 2D Fourier transform signal will cross the filter plane. It is on this second spatial light modulator that the reference template corresponding to the searched object (the target) will be displayed. In fact, it is the Fourier transform of the reference template that is recorded. So after travelling trough this second spatial light modulator, a multiplication of two Fourier transforms is obtained. In the spatial domain this corresponds to a correlation. In order to achieve the conversion between the frequency and the spatial domains, a second Fourier lens is used and the beam exits the optical system in a parallel way. The camera is the last component of the correlator and detects the intensity all over the correlation plane. Basically, the system processing speed is limited only by the refresh rate of the electro-optic components (spatial light modulator, camera), because the computation itself is performed using the light.
The optical correlator principle has been known since the work of Vander Lugt. Since then, a lot of work has been spent on generating filters to enhance specific recognition performances such as multiple target recognition with composite filters, enhanced discrimination with phase-only filters, or rotation invariant recognition with circular harmonic filters. Various optical correlator types have also been proposed such as a Vander Lugt correlator. In this correlator architecture, similar to the one illustrated in FIG. 7, the image is displayed in the input plane whereas the filter is displayed in the frequency plane. The correlation is acquired at the output plane. The filter was at that time recorded on a spatial carrier. A Joint Transform correlator (JTC) was also proposed. In a JTC, both the image and the reference template are recorded in the input plane. The interference pattern is recorded in the frequency plane and sent back to the input plane to obtain the correlation in the frequency plane, after a second pass through the correlator. Despite extended work on optical correlator filters and architectures, it did not result in solutions which address the critical opto-mechanical structure required to obtain satisfactory optical correlation performances.
Various architecture implementations have been proposed for optical correlators, such as “Coherent Optical Correlator” (U.S. Pat. No. 4,277,137), and the optical correlator principle taught in “Holographic Information Storage and Retrieval” (U.S. Pat. No. 3,608,994). Architectures have also been proposed to make the overall system more compact, such as “Compact 2F Optical Correlator” (U.S. Pat. No. 5,073,006).
These solutions usually result in optical set-ups where each individual optical element is inserted in a holder fixed on an optical table. This results in excessive production cost.
Furthermore, although optical correlator architectures were addressed in these patents, little or no consideration was devoted to the opto-mechanical structure that influences production cost and ease of alignment.
Nowadays, optical correlators are not widely spread either in terms of commercial applications or availability as commercial products. This is mainly due to the high production cost related to the aforementioned opto-mechanical structure and to the difficulty of alignment of the optical correlator.
Lack of market penetration has also left unaddressed other considerations of optical correlation implementation, such as heat dissipation and heat stabilization.
The possibility to achieve multichannel optical correlators has been addressed in U.S. Pat. No. 3,802,762. However, this possibility is limited by the availability of powerful laser sources that can drive multiple correlators simultaneously and by the interference that can be produced between the various channels.